Tasmania's House of Assembly has passed a bill to allow gay couples to marry.

After more than four hours of debate on Thursday, the bill was passed 13 votes to 11.

Long-serving Speaker Michael Polley was the only Labor MP to vote against the legislation and was joined by all 10 Liberal MPs.

Premier Lara Giddings congratulated the Labor and Greens MPs who spoke in support of the move.

"And I too look forward to being able to celebrate engagement parties, weddings, attending receptions," she said.

"I think there's a couple of people who are actually thinking of becoming celebrants. We're good talkers, us politicians."

Greens leader Nick McKim said it was a historic moment.

"The Parliament has just excelled itself," he said.

"It's placed Tasmania as a national leader on a really important issue, a progressive issue, and I'm just so proud to be a member of a progressive government and tonight so proud to be a member of the Tasmanian Parliament."

Earlier, Ms Giddings opened the debate by detailing letters from supporters.

Gay and lesbian couples had gathered in the public gallery to hear the bill debated.

Ms Giddings said she had been flooded with letters of support from all over the world.

"Daughters wrote to me in support of their lesbian mothers, mothers and fathers wrote to me in support of their children in same-sex relationships, men and women in long-term committed relationships wrote to me with the hope that soon their relationship will receive the recognition they've longed for for decades," she said.

Greens MP Kim Booth, meanwhile, launched a scathing attack on Liberal Party members for remaining silent during the debate.

Opposition Leader Will Hodgman was the only Liberal to speak, telling the House his party was unified against the bill.

He said he had never denied Liberal Party members a conscience vote on this issue.

But Mr Booth said Mr Hodgman's colleagues had betrayed their constituents by their silence and should hang their heads in shame.

While the bill has passed the Lower House, it will have a harder path through the Upper House where some MPs are yet to declare their support.

The Upper House will not consider the bill until the end of next month.